1. Field of the Invention
The present invention is related to static VAR generators that are used to provide reactive compensation to AC electrical networks and especially static VAR generators employing malfunction detectors.
2. Background
Static VAR generators (SVG's) having capacities of several hundred megavolt-amperes are used by electric utilities to preserve the stability of electric power transmission lines. Assurance is needed that the controllers of the static VAR generators are available for system use. Most controllers for SVG's utilize a feedback signal proportional to the output current, output voltage, or both in a closed loop control configuration. By comparing the difference between the feedback signal and a reference signal, representing voltage or power, the output of the SVG is made to track the reference signal. The controllers also incorporate integral control techniques so that over time any error between the output signals and the reference signals tend to be reduced to zero. The output from the integral controllers which represents the amount of error present is sent to the firing angle controllers which determine the firing or gating of the thyristor switches used in the SVG. Control over the output is accomplished by advancing or retarding the firing angle depending on the magnitude and direction of error in the output signal. An example of such a SVG utilizing these control techniques can be found in U.S. Pat. No. 4,172,234 entitled "Static VAR Generator Compensating Control Circuit and Method for Using Same" issued Oct. 23, 1979 to Gyugyi et al.
SVG's connected to the power transmission line provide voltage support or regulation during disturbances by regulating the line voltage with a closed loop control. A typical control implementation contains an adjustable voltage reference, V.sub.REF, compared against the transmission line voltage, V.sub.T, measured at the output terminals of the SVG. The resulting voltage error signal is then used to vary the reactive output of the SVG in a closed loop manner. Another feature generally required in a SVG based AC voltage regulator is voltage regulation according to a V-I (voltage-current) curve. The slope of this V-I curve has a dual purpose. First, it provides an inherent tendency to share the output power between voltage regulators operated in parallel; and second it simultaneously extends the linear control range of compensation while permitting larger system voltage variations.
One method to achieve this V-I slope control is by using a proportional controller where the closed loop gain and the resulting slope is known explicit mathematical function of the open loop gain of the regulator shown in Equation 1. ##EQU1## where G.sub.CL and G.sub.OL indicate closed and open loop gain values, respectively. The required slope setting can thus be achieved by adjusting the open loop gain of the regulator in the electronic control circuit. The open loop gain of the SVG-based voltage regulator also depends on a complex and generally unknown variable--the transmission line impedance, L.sub.XM, seen at the output terminals of the SVG. Because of this open loop variation in impedance, the required accuracy for the slope setting of the V-I curve cannot be reliably insured. However, an accurate V-I slope setting control scheme that is inherently immune from the open loop control gain variations can be constructed. In this scheme, the voltage reference V.sub.REF is continuously offset by varying the amount that is linearly proportional to the average value of the SVG output current I.sub.T as measured at the output terminals of the SVG. A slope controller having as inputs a reference slope setting S and the SVG output current I.sub.T produces a voltage signal SI that is compared to the reference signal V.sub.REF generating a voltage error signal, V*. With appropriate scaling, the error signal V* can exactly represent the required regulation slope of the SVG. V* is used as an internal reference for the controller of an SVG-based voltage regulator, the controller having a practically infinite steady-state gain implemented by 3 mode control (proportional-integral-derivative control). The main feature of the controller is that it operates without theoretical steady-state error (V*.fwdarw.0) and can therefore regulate the output terminal voltage V.sub.T exactly as called for by the input signal V* that already incorporates the required slope. The integrated error voltage output of the controller is fed to the output section of the SVG controller a firing angle converter and a firing angle control (FAC) that operate the power thyristor circuit of the SVG. The control system is illustrated in FIG. 1. Using the 3 mode control, it is possible to directly parallel SVG's at the same or nearby transmission line terminals.
With an operating SVG it is extremely difficult to determine if the SVG is operating properly due to the effect of the unknown varying impedance of the electrical network. When observing the control action, the answer to the question is the system acting in response to a disturbance in the system or to a malfunction in the control system is not always readily determined. For an individual SVG comparing the control signal input of the FAC to the output can be used to detect a problem in the output section of the SVG. However this comparison does not determine if the control signal input to the FAC is correct in response to the reference signal and the feedback signal that represents the output of the SVG. Thus it would be advantageous if a means could be provided to determine if there is a malfunction and which section--the control section or output section--of the SVG is malfunctioning. Paralleling of SVG controls will not provide accurate malfunction detection. Due to component tolerances, the outputs of paralleled SVG controls having the same reference signal tend to diverge. Because of this divergence a fault indication based on an inequality between the outputs of the paralleled SVG's is not a reliable malfunction detection scheme.
It is an object of this invention to provide a malfunction detector that can distinguish between a malfunction to either the control section or output section of an SVG.